14 NORTH AMERICAN FAUNA. [no. 19. 



still iiioru nunici'ous iind larger, and have a forest growth that is more 

 uniform in character than that of the river banks. High clifis over- 

 hanging the I'iver arc of frequent occurrence. 



From Dawson to the Alaska boundary and thence to Circle the 

 country is about the same. For a long distance in the vicinity of the 

 boundary a range of high mountains is visiljle to the northward on the 

 right bank of the river. The low, rolling hills which border the upper 

 river do not quite reach Circle, l)ut are replaced by a broad, flat country 

 known as the ' Yidvon Flats,' which extends from near Circle to Fort 

 Hamlin, a distance of about 200 miles. Through the ' Flats ' the course 

 of the river breaks up into a great man}^ channels, and the islands 

 still further increase in size and number. These are composed of 

 sand and silt, in which poplars thrive better than spruces, though the 

 latter are by no means eliminated. A wild rose {Rosa cmnaniohieaf) 

 is the most abundant shrubby plant, and on the ground below it the 

 Equisetuvi is rampant. The larger islands are identical in character 

 with the mainland, and on them the spruces form quite a heavy forest, 

 with deep moss beneath. At Fort Hamlin the river narrows again 

 and flows l)etween rolling wooded hills, which are similar to many 

 farther up the river. Small streams enter the main river frequently, 

 and the timl^er is much the same; poplars, alders, and birches cover 

 the hills in dense thickets, through which spruces are sprinkled. 

 Alders arc more numerous than before. The hills var}- in height 

 from 600 to 3,000 feet, and the highest have a distinct timberline at 

 about 2,000 feet. At the mouth of the Tanana the hills become smaller 

 and the river very much wider. Here, at Fort Gibbon, Dr. Bishop 

 found the larch {LariM aiaericana) quite alnmdant. This Avas the only 

 point at which it was seen by an}^ of our party. 



The Lower Yukon beyond the Tanana is very uniform in character. 

 The banks are low and rolling and overgrown with willows and alders; 

 farther back are higher hills covered with poplars and birch; occasion- 

 ally the summits of a few hills higher than the rest are devoid of 

 trees. On the sandy islands the willow thickets are impenetrable, and 

 where a cut bank exposes a section of tliem their slender perpendicu- 

 lar trunks stand so closely as to present a solid front like a thick hedge 

 or canebrake. Thus it continues until the limit of timber is reached 

 at Andraefski, 90 miles above the mouth of the Yukon. 



This district as a whole is characterized l)v absence of southern 

 plants and animals. Among migratory birds a few have their center 

 of abundance farther south, but all the mammals are northern forms, 

 and nearly all belong to genera of circumpolar distribution.^ Plant 

 life, though quite luxuriant, is made up of only a small number of 

 hardy species. Characteristic mammals are the Fort Yukon ground 



^ The only exceptions are Synaptomys, Fiber, and Erethizon. 



